Anyone using it for main heat source?

Not I... But I know the Zim's hot springs down the from me does and man in the dead of winter with some serious cold temp that lodge is toasty warm even when your dripping wet.

I've been debating switching to it but being used to wood heat i'm worried I wont be able to get the temperatures I'm used to in the winter

I've been mulling the idea of one as well. The part that stops me is what happens when the pipe that's underground craps out? The answer is replace it. The question is again what is the particulars of the piping? Avg. lifetime?They are costly to install, will not give you the warmth of wood or gas, but are supposed to be cheaper to operate than a heatpump. The installers I've talked to don't have enough experiance with them to answer the questions.

I'm told the piping they put underground is thick enough and good enough quality it should never have to be touched after it's buried.

Some of the ranches local to me would put a piece or two of culvert pipe vertically under their water troughs and the convection was enough to keep their tanks open all winter. Geothermal heating... unless you are near a hot spring, hot spot, etc. you will need a heat pump and will just use the ground as your reservoir. IE: in the winter, its easier to pump the heat from the 55-56 degree ground than the 32F or less ambient air. Geothermal cooling... Same idea as winter, just reversed. The earth is a great heat sink at 55-66F and the heat pump can work far more effectively than in the 80F summer air. The approximate geothermal gradient that works is one degree F per thousand feet down... though it can vary depending on the area quite a bit.